btrfs: use named constant for reserved device space

There's a reserved space on each device of size 1MiB that can be used by
bootloaders or to avoid accidental overwrite. Use a symbolic constant
with the explaining comment instead of hard coding the value and
multiple comments.

Note: since btrfs-progs v4.1, mkfs.btrfs will reserve the first 1MiB for
the primary super block (at offset 64KiB), until then the range could
have been used by mistake. Kernel has been always respecting the 1MiB
range for writes.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ update changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -229,6 +229,13 @@ struct btrfs_root_backup {
 #define BTRFS_SUPER_INFO_OFFSET			SZ_64K
 #define BTRFS_SUPER_INFO_SIZE			4096
 
+/*
+ * The reserved space at the beginning of each device.
+ * It covers the primary super block and leaves space for potential use by other
+ * tools like bootloaders or to lower potential damage of accidental overwrite.
+ */
+#define BTRFS_DEVICE_RANGE_RESERVED			(SZ_1M)
+
 /*
  * the super block basically lists the main trees of the FS
  * it currently lacks any block count etc etc

fs/btrfs/extent-tree.c

@@ -5976,7 +5976,7 @@ int btrfs_error_unpin_extent_range(struct btrfs_fs_info *fs_info,
  */
 static int btrfs_trim_free_extents(struct btrfs_device *device, u64 *trimmed)
 {
-	u64 start = SZ_1M, len = 0, end = 0;
+	u64 start = BTRFS_DEVICE_RANGE_RESERVED, len = 0, end = 0;
 	int ret;
 
 	*trimmed = 0;
@@ -6020,8 +6020,8 @@ static int btrfs_trim_free_extents(struct btrfs_device *device, u64 *trimmed)
 			break;
 		}
 
-		/* Ensure we skip the reserved area in the first 1M */
-		start = max_t(u64, start, SZ_1M);
+		/* Ensure we skip the reserved space on each device. */
+		start = max_t(u64, start, BTRFS_DEVICE_RANGE_RESERVED);
 
 		/*
 		 * If find_first_clear_extent_bit find a range that spans the

fs/btrfs/super.c

@@ -2272,17 +2272,13 @@ static inline int btrfs_calc_avail_data_space(struct btrfs_fs_info *fs_info,
 		avail_space = rounddown(avail_space, BTRFS_STRIPE_LEN);
 
 		/*
-		 * In order to avoid overwriting the superblock on the drive,
-		 * btrfs starts at an offset of at least 1MB when doing chunk
-		 * allocation.
-		 *
-		 * This ensures we have at least min_stripe_size free space
-		 * after excluding 1MB.
+		 * Ensure we have at least min_stripe_size on top of the
+		 * reserved space on the device.
 		 */
-		if (avail_space <= SZ_1M + min_stripe_size)
+		if (avail_space <= BTRFS_DEVICE_RANGE_RESERVED + min_stripe_size)
 			continue;
 
-		avail_space -= SZ_1M;
+		avail_space -= BTRFS_DEVICE_RANGE_RESERVED;
 
 		devices_info[i].dev = device;
 		devices_info[i].max_avail = avail_space;

fs/btrfs/volumes.c

@@ -1403,12 +1403,7 @@ static u64 dev_extent_search_start(struct btrfs_device *device, u64 start)
 {
 	switch (device->fs_devices->chunk_alloc_policy) {
 	case BTRFS_CHUNK_ALLOC_REGULAR:
-		/*
-		 * We don't want to overwrite the superblock on the drive nor
-		 * any area used by the boot loader (grub for example), so we
-		 * make sure to start at an offset of at least 1MB.
-		 */
-		return max_t(u64, start, SZ_1M);
+		return max_t(u64, start, BTRFS_DEVICE_RANGE_RESERVED);
 	case BTRFS_CHUNK_ALLOC_ZONED:
 		/*
 		 * We don't care about the starting region like regular